The present invention relates to a semiconductor light emitting device containing a semiconductor light emitting element, in particular a semiconductor laser diode. The present invention can be successfully adopted in applications where an excellent heat spreading ability of the semiconductor light emitting element is an important characteristic, for example in an excitation light source for optical fiber amplifiers and a light source for optical information processing in which high output and high reliability must be ensured. The present invention is also suitable for the cases where easy attainment of both of excellent heat spreading ability of the semiconductor light emitting element and direct coupling of this with an optical fiber is desired.
Remarkable progress has been made in recent technologies in optical information processing and optical fiber communication.
For example, in the communication field, extensive research efforts are directed to large-capacity optical fiber transmission and an optical fiber amplifier doped with a rare earth ion such as Er3+ (EDFA), which is expected to have the flexibility as a signal amplifier needed for a multi-terabit transmission system. Thus the development of a high-efficiency semiconductor laser diode for an excitation light source, indispensable as an EDFA component, is greatly anticipated.
An excitation light source for EDFA may, in principle, have three possible oscillation wavelengths: 800 nm, 980 nm and 1480 nm. It is known that due to the characteristics of this amplifier the excitation at 980 nm is the best with regard to gain and noise. For a laser diode of excitation light source oscillating at 980 nm, there are conflicting requirements for high output and for long life. In the wavelength range around 980 nm, there is strong needs for development of new laser diodes excellent in output power and reliability, since for example expected applications in the 890 to 1150 nm range include SHG (secondary harmonic generation) light sources, heat source for laser printers, and excitation light sources for optical fiber amplifier such as a state-of-the-art TDFA (thulium-doped fiber amplifier).
In the field of information processing, recent trends prefer higher output and shorter wavelength semiconductor laser diodes in order to achieve higher density storage and faster read/write operation. There is a strong need for higher output from conventional laser diodes (simply referred to as xe2x80x9cLDxe2x80x9d, hereinafter) having an oscillation wavelength of 780 nm, and extensive research on an LD capable of emitting light of 630 to 680 nm is being carried out from every aspect.
As for semiconductor laser diode of a 980-nm range, extensive research has been done and has resulted in practical achievement such as used in a large-capacity submarine cable systems for optical communication between Japan and the US. The reliability thereof, however, is still not satisfactory since rapid degradation may occur in the operation at higher output levels. The same applies to LD""s operating at a 780-nm range and 630- to 680-nm range.
One possible cause for poor reliability is thermal influence. Even high-efficiency models of foregoing semiconductor laser diodes can convert input electric power into light only at an efficiency of about 50%, with the rest of the electric power input lost as heat. This means that in cases where particularly high output is desired, the heat generated in semiconductor laser diode will result in remarkably declined maximum light output, degraded laser efficiency and degraded linearity in current-versus-light output characteristics. It is feared that unless there is adequate heat radiation during high-power operation, reliability will be degraded.
generally, in semiconductor laser diodes, heat spreading is ensured by soldering one electrode plane of the laser diode to a heat sink called xe2x80x9csub-mountxe2x80x9d which is typically made of AlN or Si. In this specification, an integrated structure comprising a semiconductor light emitting element (for instance LD) and the sub-mount functioning as a heat sink will be referred to as COS (chip on sub-mount), hereinafter. Also in this specification, any structure comprising a semiconductor light emitting element to which is added at least a heat spreading function will be described as a semiconductor light emitting device. The foregoing COS is therefore one kind of semiconductor light emitting device and can be incorporated into a can package or a butterfly package. Such packages are semiconductor light emitting devices with additional functions.
For fabrication of can packages, it is a general practice that a COS is mounted on a so-called xe2x80x9cstemxe2x80x9d providing further heat spreading and current injection, wirings necessary for the current injection are done, and a cap with a window seals in e.g. a nitrogen atmosphere, to thereby complete a semiconductor light emitting device. On the other hand, butterfly packages can be constructed by mounting a COS on a so-called xe2x80x9cOSA (optical sub-assembly)xe2x80x9d providing heat spreading and integrating a plurality of parts including a photo diode (PD) and then optically coupling the semiconductor light emitting element with an optical fiber etc., thereby completing the semiconductor light emitting device.
In these two cases, a semiconductor light emitting element is generally brought into contact for heat spreading only on one plane of electrode. A structure allowing the substrate side of the semiconductor light emitting element to contact with the heat sink is called xe2x80x9cjunction-up (face-up)xe2x80x9d; and a structure allowing the epitaxial layer side of the element to contact with the heat sink is called xe2x80x9cjunction-down (face-down)xe2x80x9d.
The junction-up mounting is simple and widely practiced since the method allows the light emission point of the element to be removed from the heat sink, i.e. the sub-mount, approximately by the thickness of the element. The method is, however, disadvantageous in terms of heat spreading since the light emission portion of the element is located distant from the heat sink, and so is not always suitable for high-power operation of the semiconductor light emitting element such as the semiconductor laser diode.
On the other hand, the junction-down mounting is advantageous in term of the heat spreading, but still the heat spreading is insufficient, and improvement of this is now urgently required.
Several proposals have been made for the further improvement in heat spreading of the semiconductor light emitting element, and more specifically, semiconductor laser diode. For example, Japanese Laid-Open Patent Publication No. 306681/1990 discloses a method of ensuring heat spreading of the semiconductor laser diode simultaneously in the upper and lower directions. Similar methods are also found in Japanese Laid-Open Patent Publication Nos. 228044/1996 and 228045/1996. It is, however, difficult to fabricate the disclosed structure with an excellent reproducibility by any of these methods.
This is because there is no consideration at all given to dimensional errors generally found in the individual components, typically error in the thickness of the semiconductor light emitting element, or dimensional error of the heat sink sandwiching the semiconductor light emitting element.
In a general fabrication of the semiconductor laser diode, a substrate of as thick as approximately 350 xcexcm is used to thereby ensure mechanical strength sufficient for executing necessary processes, and the substrate is later polished to reduce the thickness thereof to as thin as 100 to 150 xcexcm before the n-electrode forming process or cleavage process in order to facilitate the cleavage. It is, however, quite natural that the dimensional error in the thickness as much as 5 to 15 xcexcm is produced, which causes further error in elements. Process errors can occur also in metal components for heat spreading described in the foregoing Japanese Laid-Open Patent Publication No. 306681/1990; and recessed heat spreading components described in Japanese Laid-Open Patent Publication Nos. 228044/1996 and 228045/1996. Thus fabricating such structures disclosed in these patent applications with good reproducibility will entail a great difficulties when errors in the assembly are taken into account.
Joining by any means necessary the heat spreading component with the semiconductor light emitting element under such unstable conditions may produce strain in both due to the dimensional error and may adversely affect the reliability characteristics of the element to a large extent.
Japanese Laid-Open Patent Publication No. 340581/1999 discloses another method which is designed to ensure heat spreading upward and downward, taking reproducibility in the assembly and practicability into account. Since a laser diode is provided on the rear facet thereof with a spacer layer for controlling the level of the light emission plane, the method is successful in providing the upward and downward heat radiation mechanism in a precise manner. This is, however, disadvantageous in that it makes impossible monitoring of the light, which is generally done from the rear facet, since the spacer shadows that rear facet.
Generally constitution of the package of the semiconductor light emitting element incorporates a photo diode where a slight amount of light emitted from its rear facet is converted into electric current so as to control or monitor the light emitted from the front facet. The method disclosed in Japanese Laid-Open Patent Publication No. 340581/1999 inevitably presents the problem that such function can no longer be carried out.
Another problem resides in that, in any of these methods, direct optical coupling of the laser diode and an optical fiber can be done only when a high level of precision is ensured for the mounting of the semiconductor laser diode, in particular with the sub-mount which is mounted on the light emission point side. The optical fiber and the semiconductor laser diode must be kept with in a distance of 2 to 3 xcexcm in their direct optical coupling, which means that the semiconductor laser diode must be mounted on the end face of the sub-mount with an equivalent precision. If the front facet of the semiconductor laser diode is removed from the end face to the rear of the sub-mount by more than 2 to 3 xcexcm, sub-mount end face physically presents the end of the optical fiber from being connected at the local point. On the contrary, if the front facet of the semiconductor laser diode is positioned to the forward of the edge of the sub-mount, heat generated from the semiconductor laser diode cannot be absorbed by the sub-mount, which will have serious impact on the reliability of the element.
Considering the foregoing problems in the prior art, it is therefore an object of the present invention to provide a semiconductor light emitting device which ensures excellent heat spreading function of a semiconductor light emitting element, allowing simple and highly reproducible assembly of components and elements composing the semiconductor light emitting device even if such components and elements have intrinsic dimensional errors, and allowing simple optical coupling with an optical fiber.
The present inventors found out after extensive investigational efforts that an excellent semiconductor light emitting device showing desired effects can be obtained by sandwiching a semiconductor light emitting element with two heat sinks from the top and bottom to thereby limit the joining of both heat sinks within a specific space.
That is, the present invention is to provide a semiconductor light emitting device comprising at least one semiconductor light emitting element of edge-emission type, a first heat sink and a second heat sink, wherein at least a part of an electrode for first-conduction-type semiconductor of the semiconductor light emitting element is in contact with the first heat sink; at least a part of an electrode for the second-conduction-type semiconductor of the semiconductor light emitting element is in contact with the second heat sink; and the first heat sink and the second heat sink are in contact with each other in a junction overlooking one of the two side planes which do not compose facets of the cavity of the semiconductor light emitting element.
Preferable embodiments of the present invention include such that a portion of the electrode for the first-conduction-type semiconductor of the semiconductor light emitting element is not in contact with the first heat sink in the vicinity of the front facet of the element, and a portion of the electrode for the second-conduction-type semiconductor of the semiconductor light emitting element is in contact with the second heat sink in the vicinity of the front facet of the element; such that the plane of the first heat sink which is kept in contact with the semiconductor light emitting element has an effective electro-conductivity with at least one plane which is not kept in contact with the semiconductor light emitting element; such that the plane of the second heat sink which is kept in contact with the semiconductor light emitting element has no electro-conductivity with any planes which are not kept in contact with the semiconductor light emitting element; such that the diameter of a lead wire for introducing electric current to the semiconductor light emitting element and which is kept in contact with at least one of the group consisting of semiconductor light emitting element, the first heat sink and the second heat sink is 35 xcexcm or less, and a pair of portions not connected directly with each other are connected with each other with a plurality of lead wires; such that a space is provided in the vicinity of the junction of the first heat sink and the second heat sink, into which an adhesive used for joining such first heat sink and the second heat sink can flow to thereby prevent such adhesive from reaching the semiconductor light emitting element; such that at least a part of the electrode for the first-conduction-type semiconductor is in contact with the first heat sink as being interposed with a first adhesive, at least a part of the first heat sink is in contact with the second heat sink as being interposed with a second adhesive, and the total weight of the second adhesive is twice or above, and more preferably five times or above, heavier than the total weight of the first adhesive; such that at least one of the electrodes of the semiconductor light emitting element has an Au layer having a thickness of 30 to 100 nm; such that the first conduction type is p-type, and the second conduction type is n-type; such that the semiconductor light emitting element is a semiconductor laser diode, and the front facet thereof is provided with an optical fiber so as to compose a semiconductor laser module; and such that the tip of the optical fiber has a light focusing function, and is processed so as to be coupled directly with the front facet of the semiconductor laser diode.